The present invention relates to collagen producing plants and methods of generating and using same. More particularly, the present invention relates to a novel approach for generating plants capable of producing high levels of hydroxylated collagen chains which are capable of forming native triple helix type I collagen fibers.
Collagens are the main structural proteins responsible for the structural integrity of vertebrates and many other multicellular organisms. Type I collagen represents the prototypical fibrillar collagen and is the major collagen type in most tissues.
Type I collagen is the predominant collagen component of bone and tendon and is found in large amounts in skin, aorta, and lung. Type I collagen fibers provide great tensile strength and limited extensibility. The most abundant molecular form of type I collagen is a heterotrimer composed of two different alpha chains [alpha 1(I)]2 and alpha 2(I) (Inkinen, 2003). All fibrillar collagen molecules contain three polypeptide chains constructed from a repeating Gly-X-Y triplet, where X and Y can be any amino acid but are frequently the imino acids proline and hydroxyproline.
Fibril forming collagens are synthesized as precursor procollagens containing globular N- and C-terminal extension propeptides. The biosynthesis of procollagen is a complex process involving a number of different post-translational modifications including proline and lysine hydroxylation, N-linked and O-linked glycosylation and both intra- and inter-chain disulphide-bond formation. The enzymes carrying out these modifications act in a coordinated fashion to ensure the folding and assembly of a correctly aligned and thermally stable triple-helical molecule.
Each procollagen molecule assembles within the rough endoplasmic reticulum from the three constituent polypeptide chains. As the polypeptide chain is co-translationally translocated across the membrane of the endoplasmic reticulum, hydroxylation of proline and lysine residues occurs within the Gly-X-Y repeat region. Once the polypeptide chain is fully translocated into the lumen of the endoplasmic reticulum the C-propeptide folds. Three pro-alpha chains then associate via their C-propeptides to form a trimeric molecule allowing the Gly-X-Y repeat region to form a nucleation point at its C-terminal end, ensuring correct alignment of the chains. The Gly-X-Y region then folds in a C-to-N direction to form a triple helix.
The temporal relationship between polypeptide chain modification and triple-helix formation is crucial as hydroxylation of proline residues is required to ensure stability of the triple helix at body temperature, once formed, the triple helix no longer serves as a substrate for the hydroxylation enzyme. The C-propeptides (and to a lesser extent the N-propeptides) keep the procollagen soluble during its passage through the cell (Bulleid et al., 2000). Following or during secretion of procollagen molecules into the extracellular matrix, propeptides are removed by procollagen N- and C-proteinases, thereby triggering spontaneous self-assembly of collagen molecules into fibrils (Hulmes, 2002). Removal of the propeptides by procollagen N- and C-proteinases lowers the solubility of procollagen by >10000-fold and is necessary and sufficient to initiate the self-assembly of collagen into fibers. Crucial to this assembly process are short non triple-helical peptides called telopeptides at the ends of the triple-helical domain, which ensure correct registration of the collagen molecules within the fibril structure and lower the critical concentration for self-assembly (Bulleid et al., 2000). In nature, the stability of the triple-helical structure of collagen requires the hydroxylation of prolines by the enzyme prolyl-4-hydroxylase (P4H) to form residues of hydroxyproline within a collagen chain.
Plants expressing collagen chains are known in the art, see for example, U.S. Pat. No. 6,617,431 and (Merle et al., 2002, Ruggiero et al., 2000). Although plants are capable of synthesizing hydroxyproline-containing proteins the prolyl hydroxylase that is responsible for synthesis of hydroxyproline in plant cells exhibits relatively loose substrate sequence specificity as compared with mammalian P4H and thus, production of collagen containing hydroxyproline only in the Y position of Gly-X-Y triplets requires plant co-expression of collagen and P4H genes (Olsen et al, 2003).
An attempt to produce human collagens that rely on the hydroxylation machinery naturally present in plants resulted in collagen that is poor in proline hydroxylation (Merle et al., 2002). Such collagen melts or loses its triple helical structure at temperatures below 30° C. Co-expression of collagen and prolyl-hydroxylase results with stable hydroxylated collagen that is biologically relevant for applications at body temperatures (Merle et al., 2002).
Lysyl hydroxylase (LH,EC 1.14.11.4), galactosyltransferase (EC 2.4.1.50) and glucosyltransferase (EC 2.4.1.66) are enzymes involved in posttranslational modifications of collagens. They sequentially modify lysyl residues in specific positions to hydroxylysyl, galactosylhydroxylysyl and glucosylgalactosyl hydroxylysyl residues. These structures are unique to collagens and essential for their functional activity (Wang et al, 2002). A single human enzyme, Lysyl hydroxylase 3 (LH3) can catalyze all three consecutive steps in hydroxylysine linked carbohydrate formation (Wang et al, 2002).
Hydroxylysins of a human collagen expressed in tobacco form less than 2% of the hydroxylysins found in a bovine collagen (0.04% of residues/1.88% of residues). This suggests that plant endogenic Lysyl hydroxylase is unable to sufficiently hydroxylate lysines in collagen.
While reducing the present invention to practice, the present inventors uncovered that efficient hydroxylation of collagen chains relies upon sequestering of the collagen chain along with an enzyme capable of correctly modifying this polypeptide.